Status of Cryogenics at point 4 following the power cut

1. Status of Cryogenics at point 4 following the power cut L. Tavian 20 August 2011

2. The issue • Problem with one cold compressor (CC2) on the pumping unit of P4, S4-5. • Trips of the compressor during the pump-down • Overload on magnetic bearing (still to be investigated and confirmed) • The cause of this trip is not yet identified (electronics, powering, cabling, mechanics…)

3. CC2 speed trend (Hz) Electrical power cut Nominal speed 4 trips during pump-down

4. Normal action plan • Reset • Remotely if process trip (PVSS action) • Locally if “assisted landing” • Locally with recalibration if “hard landing” • Replace • Active magnetic bearing drive • Frequency drive and other electronics • Transformer with check of the cabling to the motor • CC cartridge

5. Electrical cabinet of a cold compressor Magneticbearing drive Frequency drive Warner K4000 Transformer 400 V - 150 V

6. Cold compressor cartridge

7. Normal action plan • Reset • Remotely if process trip • Locally if “assisted landing” • Locally with recalibration if “hard landing” • Replace • Active magnetic drive • Frequency drive and other electronics • Transformer with check of the cabling to the motor • CC cartridge NOK then OK

8. Present situation now OK (25 mbar Cryo-start around 3 a.m.) 1 trip Last intervention

9. Future plan • If the present run not successful  continue the normal action plan (Plan A): • Exchange the transformer (spare already in UX45) (~1 h interv.) • Exchange the CC cartridge: • Warm-up of the cold box on Sunday nigth • Exchange of cartridge on Monday morning (5-8 h) • Cool-down of the cold box on Monday (8 - 10 h) • Validation test (2-3 h) Tuesday morning • Pump-down to cryo-start (24 h) • Remark: As the cause of the problem is not clearly identified the success of these interventions is not known

10. Other alternative (Plan B) • Limit the maximum speed of this compressor below the trip threshold (260 Hz ?). • Need of the CERN expert on Linde CC PLC  not before Tuesday (8h intervention) • Try to assess which pressure can be reached by pushing the other stages. • Difficult to assess as the cold compressors are limited by their control algorithms (black box). • Personal remark: limited confidence in reaching stable cryo-maintain condition with this alternative

11. Other alternative (Plan C) • Use redundancy with S3-4 cryoplant • Because of RF load at P4, not possible to implement the P8 and P6 configuration (one cryoplant on 2 sectors) • Because of RF at P4, not possible to implement the P2-P18 configuration (thermal shields on the S4-5 cryoplant and the rest (CM, BS, DFB, RF…) on the S3-4 cryoplant (low-load ex-LEP plant). • Implement another redundant scheme: • Supply of the 1.9 K cooling loop via the S4-5 cryoplant • Return of the pumping flow via the S3-4 cryoplant • Consequences: • The S4-5 cryoplant will have and additional liquefaction duty of about 50 g/s (seems to be compatible with its capacity margin) • The S3-4 cryoplant will have an extra cold return flow (~50 g/s) at 20 K and will work as “economizer cycle”  turbine attenuation. • Personal remark: The better alternative plan with nominal conditions untilthe next TS

12. Summary • Continue the present pump-down  CS by 3 a.m. • If trip again: change of the transformer • If trip again: start plan C on Sunday morning • CS expected ~40 h after restart(but it is a new operation mode with possible surprises) • Wait the next TS for cartridge exchange and/or further investigations.